This invention relates to fuel cells and, in particular, to a method of power load recovery in fuel cell systems.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
Fuel cells operate by passing a reactant fuel gas through the anode, while oxidizing gas is passed through the cathode. The electrical output of the fuel cell system depends in part on the rates at which humidified fuel gas and the oxidizing gas are supplied to, and are carried through, the anode and the cathode, respectively. In order to increase or ramp the power output of the fuel cell system from a low load to a high load corresponding to a high target power output, the rates at which the humidified fuel gas and the oxidant gas are supplied to the fuel cell system are correspondingly increased.
High temperature fuel cell systems, such as molten carbonate fuel cells (“MCFCs) and solid oxide fuel cells (“SOFCs”), are capable of operating at high power outputs of about 250 kW or higher. However, when the power output or load on the fuel cell system changes, the thermal profile of the system is also caused to change in response to the changing electrochemical reaction rates, imposing thermal and mechanical stresses on the fuel cell stack. In order to maintain a relatively constant thermal profile within the fuel cell stack and to minimize thermal-mechanical stresses on the stack, increases in the power output and thus the load on the system typically must be made at a slow and controlled rate. For example, the power output for a MCFC system operating at a low load is typically increased at a rate of 0.5 kW/min until the system reaches the high target power output. As a result, a fuel cell system requires a significant amount of time in order to reach high load operating conditions without causing harm the fuel cell stack.
In certain circumstances, a fuel cell system operating at high load conditions may suddenly drop from the high load to a low load. Such sudden drop in the load and thus the power output may be caused by a short term grid disruption in which the fuel cell system connected to a power grid becomes separated from the grid, thus eliminating the load source created by the grid and reducing the load on the system to an island load. In other cases, the sudden drop in the power output is commanded by a controller to allow a process change, such as fuel switching, or to address a failure of a system component. This sudden commanded drop in power output may result in a low load condition or a zero load condition.
Upon the return of the system's ability to ramp to a high load, the conventional method is to increase, or ramp, the power output of the fuel cell at a slow gradual rate from the low load to the high load, terminating at the high target power output. The high target power output is usually equivalent to the previous high load. The conventional method requires a significant amount of time, and therefore results in a considerable reduction in the system's electrical output and efficiency. Accordingly, a rapid recovery of the load to allow the fuel cell system to quickly return to a high load operating conditions after the disruption without causing harm to the fuel cell stack is desired.
It is therefore an object of this invention to provide a method for rapid recovery of high load conditions in the fuel cell system after a momentary drop of the load from high to low.
It is a further object of the invention to provide a method for rapid load recovery that minimizes thermal and mechanical stresses on the fuel cell stack and does not result in damage to the stack.